requests library throws InvalidHeader because 'Content-Length' is int

Couple of months ago, we started seeing this exception in our logs:
```
InvalidHeader("Header value 556 must be of type str or bytes, not <type 'int'>",)
```
Turns out it's the `Content-Length` header in `__init__`, which is int; it should be converted to type `str`.

PR which fixes the issue is ready to merge: #14. 